Field devices are used in industrial process monitoring and/or control systems to monitor process parameters associated with a particular process. Such process parameters can include fluid pressure, fluid flow rate, fluid temperature, other process variables, or any combination thereof.
Magnetic flow meters are a type of field device that is used to measure a fluid flow rate of a conductive process fluid as it flows within a flow tube that is coupled to a pipe. A particular magnetic flow meter includes an electromagnet and electrodes. In accordance with Faraday's law of electromagnetic induction, the electromagnet is used apply a magnetic field to the process fluid within the flow tube to induce an electromotive force (EMF) in the process fluid, which EMF is proportional to the flow rate. The electrodes are positioned in the flow tube to make electrical contact with the flowing process fluid to sense the EMF. In a particular embodiment, the EMF is measured by the flow meter using an amplifier connected across the electrodes to amplify the EMF signal and using an analog-to-digital converter (ADC) to quantize the output of the amplifier to produce a data value related to the fluid flow rate. In a particular example, the EMF is measured relative to a signal ground, which may be tied to the process fluid. The electrical potential of the process fluid can be used as a reference, which may not necessarily represent an Earth ground connection.
The magnetic flow meter and associated transmitter circuitry are referenced to the process to provide a stable reading. This process or ground connection is established via an electrical connection between the flow tube and the process. The ground connection can be established using ground rings, which strap to the flow tube, a ground electrode which is connected directly to the flow tube, a strap between the flow tube and the adjacent conductive pipe, another ground connection, or any combination thereof. In a particular embodiment, earth ground can provide a low noise reference and often is required by electrical safety code. However, poor ground connections can lead to erroneous process measurements.